In recent years, electronic devices have been using components such as CPUs (Central Processing Units), memory, micro controllers, and the like that require various power supply voltages, and accordingly such electronic devices include a power supply device that outputs a plurality of power supply voltages. When a centralized-type power supply device, in which an external voltage is converted by a single stabilized-power supply into a plurality of power supply voltages to be output to components, is used, wiring resistance between the stabilized-power supply and the components causes a voltage drop, decreasing the power supply efficiency. Also, when load currents flowing through components vary, the amount of voltage drop caused by wiring resistance varies, thus varying the power supply voltages output to the components.
In order to cope with this, a method is known in which a stabilized-power supply is used to step down an external voltage into an intermediate voltage, and a plurality of distributed power supplies to convert an intermediate voltage into a power supply voltage are each disposed in immediate proximity to a component in order to reduce the voltage drop caused by wiring resistance. This distributed power supply is called a POL (Point of Load) power supply. Also, a non-stabilized power supply that yields a high supply efficiency is sometimes used instead of a stabilized-power supply so as to step down an external voltage to an intermediate voltage and to convert the intermediate voltage into a plurality of power supply voltages by using a plurality of POL power supplies. When an intermediate voltage output from the non-stabilized power supply varies, each of the plurality of POL power supplies performs stabilizing operations.
A switching power supply capable of outputting a low voltage and a high current is sometimes used for a POL power supply. A switching power supply performs switching on an external voltage in accordance with a pulse signal so as to output a power supply voltage, and also performs feedback control on the duty ratio of the pulse signal so that the power supply voltage to be output becomes consistent. Thereby, the switching power supply suppresses variations in a power supply voltage so as to stabilize the power supply voltage (See Patent Documents 1 through 3 for example).
Patent Document 1: Japanese Laid-open Patent Publication No. 2002-315313
Patent Document 2: Japanese Laid-open Patent Publication No. 2004-147371
Patent Document 3: Japanese Laid-open Patent Publication No. 2005-110374
In spite of the fact described above, the feedback control of a power supply device using a conventional POL power supply is not capable of following variations in an external voltage that are quicker than the feedback delay time, thus allowing the power supply voltage to vary, which has been problematic.